In recent years, there has been a growing demand for various printed boards along with a reduction in weight, a reduction in size, and an increase in density of electronics products. In particular, there has been a rapidly growing demand for flexible laminates (herein also referred to as “flexible printed wiring boards (for FPCs)”). A flexible laminate is structured to include, on an insulating film such as a polyimide film, a circuit made of a metal layer.
The flexible printed wiring board is made originally from a flexible metal-clad laminate. In general, a flexible metal-clad laminate is produced by a method which uses, as a substrate, a flexible insulating film made of various insulating materials and that bonds a metal foil onto a surface of the substrate via various adhesive materials by heating and pressure bonding. As the insulating film, a polyimide film or the like is preferably used. As the adhesive material, a thermosetting adhesive such as an epoxy adhesive or an acrylic adhesive is generally used. A thermosetting adhesive has an advantage of allowing for adhesion at a comparatively low temperature. However, along with stricter requirements for properties such as heat resistance, bendability, and electric reliability, fabricating a three-layer FPC with use of a thermosetting adhesive is expected to involve difficulty in satisfying these requirements. For this reason, a two-layer FPC has been proposed which is obtained by providing a metal layer directly on an insulating film or whose adhesive layer is made of a thermoplastic polyimide. Such two-layer FPCs are superior in property to three-layer FPCs, and are thus expected to experience an increase in demand in the future.
An example method for producing a three-layer polyimide film for a two-layer FPC is a method of applying a polyamic acid solution to a surface of a polyimide film and drying (imidizing) the applied solution to produce a three-layer polyimide film. Such a method, however, requires a step of producing a polyimide film and a step of applying a polyamic acid solution to a surface of the polyimide film and drying (imidizing) the applied solution. The above method thus involves more than one step, and costs can be increased as a result (see, for example, Patent Literature 1).
Another example method for producing a three-layer polyimide film for a two-layer FPC is a method of cast-flowing polyamic acid solutions in a plurality of layers simultaneously onto a support, drying the cast-flown solutions, removing the dried product from the support, and heat-treating the removed product to produce a three-layer polyimide film. Such a method can, however, pose problems that (i) a polyimide layer in direct contact with the support is adhered to the support partially and (ii) there is a difference in peel strength between a polyimide layer in contact with the support and a polyimide layer present oppositely from the above polyimide layer (see, for example, Patent Literatures 2 and 3).